One of quality control methods in sheet rolling and plate rolling is automatic gauge control (AGC) which involves controlling the plate thickness of a rolled material in the middle part of the width direction. Examples of concrete control methods include monitor AGC which involves feeding back measured values of a plate thickness gauge installed on the exit side of a rolling mill, gauge meter AGC (GM-AGC) which involves using gauge meter plate thicknesses estimated from rolling loads and roll gaps (the clearance between top and bottom work rolls), and mill modulus control (MMC) which involves using rolling loads.
For example, in the case of hot rolling, temperature variations of rolled materials can be mentioned as disturbances which hinder an improvement in thickness accuracy. As disturbances common to hot rolling and cold rolling, other kinds of control items, for example, tension variations due to the deterioration of tension control, changes in speed or roll gap by an operator's manual intervention, roll eccentricity caused by accuracy deficiencies of the roll structure or roll grinding can be mentioned.
Among these disturbances, the main cause of the above-described roll eccentricity is that when key grooves of support rolls having oil bearings are subjected to a rolling load of as large as several hundreds of tons to two to three thousands of tons, shafts move up and down (undergo shaft oscillation). When roll eccentricity occurs, variations in roll gap occur correspondingly to the rotation of rolls.
Even in the case of rolls not provided with key grooves, periodic roll gap variations dependent on the rotation of the rolls occur caused by asymmetry during roll grinding and uneven thermal expansion, for example.
A rolling mill is provided with a roll gap detector for detecting roll gaps, and a device which controls roll gaps controls a screw-down device by feeding back detected values of the roll gap detector so that the roll gap obtains a given value (a set value). However, disturbances dependent on the shaft oscillation of rolls, such as roll eccentricity, cannot be detected by a roll gap detector. That is, the effect of the shaft oscillation of rolls does not manifest itself in detected values of the roll gap detector. For this reason, it is impossible to perform such control as to suppress the disturbances dependent on the shaft oscillation of rolls even when a roll gap detector is used. However, because in actuality, the disturbances dependent on the shaft oscillation of rolls change roll gaps, the effect of the shaft oscillation of rolls manifests itself in rolling loads. Therefore, the disturbances dependent on the shaft oscillation of rolls provides a great factor responsible for hindering an improvement in thickness accuracy in GM-AGC, MMC and the like which involve performing gauge control using rolling loads.
In order to reduce disturbances which periodically occur (hereinafter, referred to as “periodic disturbances”) such as roll eccentricity, roll eccentricity control has hitherto been performed. Some examples related to roll eccentricity control are described below.
In the following descriptions (including the description of the present invention), the same concept can be used in the case of what is called a 2Hi mill, which is composed of only two of the top and bottom work rolls, the case of what is called a 4Hi mill, which is composed of a total of four rolls: two of the top and bottom work rolls and two of the top and bottom support rolls, and the case of what is called a 6Hi mill, which is composed of a total of six rolls: two of the top and bottom work rolls, two of the top and bottom intermediate rolls, and two of the top and bottom support rolls, and even in the case of a mill composed of not less than six rolls. For this reason, in the following, the terms “WR” for work roll and “BUR” for back up roll, which are rolls other than work rolls, are used.
(A) Roll Eccentricity Control 1
Before the rolling of a rolled material, the top and bottom work rolls are brought into contact with each other, and the rolls are rotated, with a given load applied to the rolls (in a kiss-roll condition), and a load in the kiss-roll condition is detected. Then, roll eccentricity frequencies are analyzed by performing the fast Fourier transformation and the like of the detected load in the kiss-roll condition. During rolling, it is assumed that roll eccentricity at the analyzed frequency occur, and a manipulated variable of roll gap is outputted in such a manner as to reduce the effect of the above-described roll eccentricity without performing feedback control using loading loads.
(B) Roll Eccentricity Control 2
Plate thickness variations are measured using a plate thickness gauge installed on the exit side of a rolling mill. Then, a thickness deviation is computed linking at which rotation positions of rolls, values measured by the plate thickness gauge have been obtained during rolling. The control apparatus manipulates roll gaps according to the computed thickness deviation and reduces the thickness variations due to roll eccentricity.
(C) Roll Eccentricity Control 3
During rolling, rolling loads are detected and roll eccentricity components are extracted from the rolling loads. Then, the extracted roll eccentricity components are converted to roll gap signals, and roll gaps are manipulated so that the rolling load variations due to the roll eccentricity are reduced (refer to Patent Literature 1 and Patent Literature 2).